Sensitive noses

These 'signatures' are formed as fish constantly emit molecules into their surroundings.

Previous studies have shown fish can use visual cues to discriminate when there is a large difference in body length - more than 20 per cent between the selecting fish and those being assessed.

However he says field studies also show fish are capable of even finer discrimination than this.

"We know the sense of smell is well developed in fish and that they are sensitive to tiny differences in the chemical signature given off by others," says Ward.

Their work suggests the fish can "smell" how big they are themselves and use this chemical cue as a reference point to assess the size of others.

"Fish are strongly attracted to other fish that smell like themselves," he says. "They can smell something that gives an indication of how big a fish is."

For the study the researchers looked at two freshwater fish species - the three-spined stickleback and banded killifish.

In a series of experiments they exposed the fish to water flows containing chemical signatures of different-sized fish.

The water flows were kept separate with a "blank" stream of water separating the two streams and "typically if it preferred one fish to another [the fish] would sit in that plume [flow]", says Ward.

Both species of fish preferred the water flows that contained the chemical cues of similar-sized fish from their own species, Ward says.

The researchers have yet to pinpoint exactly what chemicals play the key role in this selection process, however Ward believes the major histocompatibility complex molecule, which plays a role in how we all smell, is involved.

Ward says the finding is relevant to all shoaling fish, however he says marine species are known to release fewer chemical cues into the water.